This invention relates generally to microwave circuits and more particularly to multilayer stripline assemblies.
Many systems employ microwave circuitry. Often, these systems must fit into small volumes, and it is desirable to reduce the size of the microwave circuitry. The space problem might, for example, be particularly acute in a power divider where signals are routed to several points. One way to reduce the volume occupied by microwave circuitry is to form several levels.
Several layers of striplines are formed in a dielectric medium which can be stacked to form the various levels. These striplines carry the microwave signals or are part of stripline circuit elements. In this way, the packing density of the circuitry is increased much the same way a multilayer printed circuit board increases the packing density of low frequency circuitry.
One problem associated with having multilevel microwave structures is that of routing signals between the various levels. For example, it is often desirable to have all inputs and outputs to the assembly on the same level to facilitate connections to the assembly. Signals then must be routed from the level with inputs and outputs to all other levels. In other instances, it may be necessary to have one signal pass from one layer to another to facilitate the desired interconnection of circuit elements.
Because microwave signals propagate in transmission lines, wires cannot simply be used to interconnect the various levels of a microwave assembly. One approach to providing interconnections between various levels in a microwave assembly is the use of coaxial feed throughs. A piece of coax, which supports propagation of microwave signals, passes through the structure perpendicular to the stripline. The coaxial feed-through is coupled at each end to the striplines on different levels.
The coaxial feed through is relatively compact. However, it suffers from what can sometimes be a significant drawback. The modes of propagation in the stripline and the coaxial feed through are different. Thus, there are two mode transitions in the feed through. Mode transitions can cause signal reflections which impair the operation of the circuit. Also, the use of coax to join two striplines may result in impedance mismatches which reduce the level of the desired signal.